ixgbe: fix for race with 8259(8|9) during shutdown

There is a small window where the watchdog could be running as the
interface is brought down on a NIC with two ports wired back to back.
If ixgbe_update_status is then called can lead to a panic.  This patch
allows the update to bail if we are in that condition.

This issue was orignally reported and fix proposed by Akihiko Saitou.

CC: Akihiko Saitou <asaitou@users.sourceforge.net>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ixgbe/ixgbe_main.c

@@ -5282,6 +5282,10 @@ void ixgbe_update_stats(struct ixgbe_adapter *adapter)
 	u32 i, missed_rx = 0, mpc, bprc, lxon, lxoff, xon_off_tot;
 	u64 non_eop_descs = 0, restart_queue = 0;
 
+	if (test_bit(__IXGBE_DOWN, &adapter->state) ||
+	    test_bit(__IXGBE_RESETTING, &adapter->state))
+		return;
+
 	if (adapter->flags2 & IXGBE_FLAG2_RSC_ENABLED) {
 		u64 rsc_count = 0;
 		u64 rsc_flush = 0;